1. Field of the Invention
The present invention relates to a reading optical system. For example, the present invention relates to a reading optical system suitable for an image reading device (particularly facsimile and digital copier) provided with a linear image sensing device such as a line CCD (charge-coupled device).
2. Description of the Related Art
Conventionally, refractive optical systems of the symmetrical type, e.g., Gauss type, Xenoter type, Orthometar type, mainly have been proposed as reading optical systems for image reading devices. There also have been various proposals for reading optical systems using an aspherical surface and these symmetrical type systems as a base. Japanese Laid-Open Patent Application No. H7-333505 discloses a reading optical system combining a reflective optical system and refractive optical system. The optical system proposed in Japanese Laid-Open Patent Application No. H7-333505 aims to provide a thin type optical system by arranging mirrors having optical power anteriorly and posteriorly to the refractive optical system, wherein the shape of the reflective surface of the mirrors is a free curve.
When meeting the demand for high speed and high density in recent years in symmetrical type optical systems, six or more lenses are inevitably required, with a resulting increase in cost. Even when using an aspherical surface on a symmetrical type system base, effectiveness beyond a certain degree cannot be obtained because the aspherical surface is formed on a refractive surface, and effectiveness is unexpectedly small when the aspherical surface is reduced to a single surface. The example cited in Japanese Laid-Open Patent Application No. H7-333505 is unclear about optical performance when the number of structural elements is greater than two mirrors and three lenses. Moreover, since it is unclear what shape the reflective surface uses, the degree of effectiveness obtained thereby is also unclear.
The present invention provides a reading optical system which reduces the number of lenses while maintaining optical performance by effectively using a free curved mirror which has a free curved reflective surface.
In one embodiment of the invention, there is a reading optical system for projecting a document image on a linear image sensing device. The reading optical system has one or more mirrors and a symmetrical lens group rotatable relative to the optical axis of the reading optical system; wherein the lens group includes sequentially from the object side a positive lens, negative lens, positive lens; the mirror faces a reflective concave surface on the object side; the shape of the reflective surface of the mirror is symmetrical relative to a plane perpendicular to the layout direction of the image sensing device and includes the intersection of the mirror reflective surface and the optical axis of the lens group; and the shape of the reflective surface of the mirror is asymmetrical relative to a plane parallel to the layout direction of the image sensing device and includes the normal line of the mirror at the intersection.
The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.